Parallel 6kW tankless water heaters for shower?

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Andy911

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Hello, I am plumbing out a seasonal cottage that needs to be winterized and I am considering the use of 2 Rheem 6kW tankless electric water heaters in parellel just to run the single shower in the cottage. The main reason I am considering this small heater is because the heating element in these is single and horizontal, it seems easy to be able to drain it for the winter. I have a brand new 200A service to the cottage so electrical draw is not a problem.

What are the main challenges with plumbing these tiny water heaters in parallel? Or do you have any other suggestions for a solution that can be drained for the off season (winterizing with antifreeze will be logistically very challenging).

Note that I have a separate (non-potable) source of water for the shower, and a separate potable source for the bathroom/kitchen sinks that will be on a separate 3rd Rheem 6kW heater.

Thanks!
 
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Fitter30

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Any tankless heater kw/ btu size should have a gpm to out going temperature to coldest incoming water temperature. It would be worth while to call Rheem ask them incoming 40° f water 120° f out what to flow rate is to see if that is satisfactory.
 

Andy911

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Well the good news in this application is the water is coming from above ground IBC tank storage during the summer, so the input water is going to be much warmer than 40, and the shower temperature does not need to get to 120. Arguably one heater may do the trick, but I’m just not sure.
 

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Andy911

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I mean, the incoming water is just going to be ambient summer temp, maybe 70-75F? The 6kW heater will lift 37 degrees at 1gpm or 25 degrees at 1.5gpm, so with a low flow head a single heater is a little marginal just on those assumptions.
 

Bannerman

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6 kW @ 240 volts = 25 amps for each

Verify the minimum flow rate required to activate each WH. With a parallel plumbing configuration, flow will be divided equally between both units. Most showers will utilize a mixture of both hot and cold water to achieve the desired temperature.

If for instance, 1 GPM hot water is flowing to a 1.5 GPM shower head, that will equal only 0.5 GPM flowing through each WH.

Since you're OK with hot water that is cooler than 120F, then a larger percentage of hot water will flow from the shower head, which will increase the flow rate through each WH.
 

Andy911

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Thanks. In this scenario you are right, the mixing valve is probably going to be mostly letting hot through. Will I have a challenge finding mixing valves that can go 100% hot, or is this normal?
 
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